Gold(i)-catalyzed C-glycosylation of glycosyl: Ortho -alkynylbenzoates: The role of the moisture sequestered by molecular sieves

Article abstract of DOI:10.1039/c6cc07218f

C-Glycosylation of glycosyl ortho-hexynylbenzoates with allyltrimethylsilane or silyl enol ethers could proceed smoothly under the catalysis of Ph₃PAuNTf₂ to provide the corresponding C-glycosides in high yields and stereoselectivity, wherein the moisture sequestered by the molecular sieves was disclosed to play a critical role in the gold(i)-catalytic cycle.

Full text of DOI:10.1039/c6cc07218f

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DOI: 10.1039/C6CC07218F
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Gold(I)‐Catalyzed C‐Glycosylation of Glycosyl ortho‐
Alkynylbenzoates, a Role of the Moisture Sequestered by
Molecular Sieves
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaoping Chen, Qiaoling Wang and Biao Yu*
DOI: 10.1039/x0xx00000x
www.rsc.org/chemcomm
C‐Glycosylation of glycosyl ortho‐hexynylbenzoates with
allyltrimethylsilane or silyl enol ethers could proceed smoothly
under the catalysis of Ph3PAuNTf2 to provide the corresponding C‐
glycosides in high yields and stereoselectivity, wherein the
moisture sequestered by the molecular sieves were disclosed to
play a critical role in the gold(I)‐catalytic cycle.
Replacement of the glycosidic C‐O acetal linkage in a native glycan
or glycoside with a C‐C linkage has become a common strategy to
attain carbohydrate mimics which are metabolically stable and thus
of therapeutic promise.1 On the other hand, C‐glycosides occur
widely as natural metabolites, which show a great structural
diversity in association with a wide spectrum of biological
activities.2 Therefore, synthesis of C‐glycosides has long been a
topic of intensive research.1,3 Among the various approaches
developed for the C‐glycosides synthesis, C‐glycosylation adopting
modification of the O‐glycosylation protocols, in that C‐nucleophiles
are used as coupling partners to condense with the conventional
glycosyl donors, constitute a straightforward alternative.3,4
Scheme 1. The gold(I)‐catalyzed O‐ and N‐glycosylation reaction
with a glycosyl ortho‐alkynylbenzoate I as donor (the counter anion
is omitted).
Recently, we developed a new glycosylation protocol that uses
glycosyl ortho‐alkynylbenzoates as donors and a gold(I) complex
(e.g., Ph3PAuOTf or Ph3PAuNTf2) as catalyst (Scheme 1).5 The mild
reaction conditions ensure the successful application of this method
in the synthesis of extremely complex O‐ and N‐glycosides.6,7
Mechanistic studies8 show that activation of the ortho‐
alkynylbenzoate donor I by a gold(I) species LAu(I) leads to
formation of glycosyloxypyrylium gold(I) intermediate III,9b,c which
may collapse into sugar oxocarbenium IV and isochromen‐4‐yl‐
gold(I) complex V.9a Gold(I) complex V is found stable, however, it
can be protonated to re‐generate the catalytic gold(I) species.
Therefore, the proton, which is released from the glycosidic
coupling of oxocarbenium IV with nucleophile HNu, plays a critical
role in the catalytic cycle of the glycosylation (I + HNu  VI + VII).
Based on this mechanistic rationale, the feasibility of a C‐
glycosylation with such commonly used C‐nucleophiles as
allyltrimethylsilane or silyl enol ethers is not affirmative, because
the regeneration of the gold(I) species via protodeauration (i.e., V +
H+  VII + LAu(I)) is no longer valid.10‐13 This mechanistic query
together with the practical importance of the C‐glycosylation
reaction prompted us to explore the C‐glycosidic coupling of
glycosyl ortho‐alkynylbenzoates with allyltrimethylsilane/silyl enol
ethers under the action of a gold(I) complex.
We first tried the C‐glycosylation of perbenzoyl and perbenzyl
glucopyranosyl ortho‐hexynylbenzoates (1a and 1b), two reliable O‐
glycosylation donors,5 with allyltrimethylsilane (2a) under the
conventional O‐glycosylation conditions (Table 1). Indeed,
treatment of 1a or 2a in the presence of Ph3PAuOTf or Ph3PAuNTf2
(0.1 equiv) (CH2Cl2, 4Å MS, RT) led to complex mixtures, wherein
the desired allyl C‐glycoside 3a was not detected at all (entries 1
and 2). Treatment of 1b and 2a in the presence of Ph3PAuOTf (0.1
equiv) also led to a complex mixture, however, the desired C‐
glycoside 3b was detectable on TLC (entry 3). Interestingly, when
Ph3PAuNTf2 (0.1 equiv) was used as the catalyst, the C‐allylation of
a.State Key Laboratory of Bio‐organic and Natural Products Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road,
Shanghai 200032, China. E‐mail: byu@mail.sioc.ac.cn
b.† Electronic Supplementary Informaꢀon (ESI) available. See DOI:
10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 2015
Chem.Commun. 2015, 00, 1‐4 | 1
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1b proceeded smoothly, leading to C‐glycoside 3b in 85% yield with provide Me3SiOH. To identify the silyl derivatives, we replaced the
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complete ‐selectivity (entry 4).4
allyltrimethylsilane with allyldiphenylmethylsilane (2b) in
DOI: 10.1039/C6CC072t1h8eF
reaction with 1b. Disiloxane (Ph2MeSi)2O was easily isolated as the
product (~93% yield based on the starting 2b), which might be
derived from the condensation of Ph2MeSiOH and Ph2MeSi+, while
condensation of Ph2MeSiOH under the action of the gold(I) catalyst
was also possible.16 On the basis of these data, the mechanism of
the present C‐glycosylation was proposed as shown in Scheme 2.
Table 1. Attempted C‐glycosylation of glucopyranosyl ortho‐
hexynylbenzoates 1a and 1b with allyltrimethylsilane (2a).
Entry Donor Ph3PAuX
Resultsa
1
2
3
4
1a
Ph3PAuOTf
Ph3PAuNTf2
Ph3PAuOTf
Ph3PAuNTf2
Complex, 3a not detectable
Complex, 3a not detectable
Complex, 3b detectable
3b (85%,  only)
1b
aIsocoumarin VII was identified as a side product, but not the
trimethylsilyl derivative VIII.
Scheme 2. The mechanistic rationale of the gold(I)‐catalyzed C‐
glycosylation of glycosyl ortho‐alkynylbenzoate I with allylsilane.
It was reported that allyltrimethylsilane (2a) was vulnerable
toward Ph3PAuOTf,14 this might explain the failure of the present C‐
glycosylation with Ph3PAuOTf as the catalyst. Indeed, NMR analysis
showed that 2a was completely decomposed (mainly into propene)
in the presence of Ph3PAuOTf (0.1 equiv) (CD2Cl2, RT) within 10 min,
whereas it remained largely intact within 30 min in the presence of
Ph3PAuNTf2 (0.1 equiv) (See SI for details). The failure of perbenzoyl
donor 1a (a disarmed donor) in the present C‐glycosylation might
be attributed to its low reactivity; decomposition of the sugar
oxocarbenium intermediates (e.g., IV) competed favorably to the C‐
allylation reaction.
What surprised us most was in the successful C‐glycosylation of
the armed donor 1b that isocoumarin VII was produced but not the
trimethylsilyl derivative VIII, which was known to be stable and
isolable.15 Indeed, treatment of the purified isochromen‐4‐yl‐gold(I)
complex V with TMSOTf did not lead to VIII. Therefore, there must
be a proton source, most likely the moisture, which enabled the
regeneration of the catalytic Au(I) species (from V) in the present C‐
glycosylation.10
To testify the scope of the present gold(I)‐catalyzed C‐
glycosylation, we first examined the reaction of a panel of the
ortho‐hexynylbenzoate donors (1c‐1i) with allyltrimethylsilane 2a
(Table 2). All the donors were consumed within 30 min in the
presence of 2a (1.2 equiv), Ph3PAuNTf2 (0.1 equiv), and 4Å MS in
CH2Cl2 at RT. The allylation reactions of 2‐deoxy‐ribofuranosyl
donors 1c and 1d led to the allyl ‐C‐glycoside 3c (80%) and 3d
(99%), respectively, in a complete stereoselective manner, although
much higher yield was attained with the more reactive benzyl
protected donor 1d (entries 1 and 2). Using 2‐deoxy‐xylofuranosyl
ortho‐hexynylbenzoate 1e as donor, the reaction at RT gave a
complex mixture (partly due to the migration of the
methoxybenzoyl group). Nevertheless, the C‐allylation proceeded
cleanly at ‐72 oC, furnishing allyl C‐glycoside 3e in good yield (88%,
α/β = 1:3) (entry 3). The 1,3‐cis‐selectivity in the C‐glycosylation of
furanosyl donors have been well studied by Woerpel et al.,17a,b and
was accounted by the preferred inside attack on the lowest energy
conformers of the intermediate oxocarbenium ions displaying the
C‐3 alkoxy group in a pseudoaxial orientation to maximize
electrostatic effects.17 Subjection of 2,3,5‐tri‐O‐benzyl‐D‐
arabinofuranosyl donors 1f or 1f to the present reaction led to
allyl C‐glycoside 3f in 80% and 87% yield, respectively, with an
identical stereoselectivity (/ = 1:2.5) (entries 4 and 5). The slight
difference in yields testified that the ‐ortho‐hexynylbenzoate
A conventional glycosylation reaction is always performed under
anhydrous conditions, wherein the dried reaction mixture is
charged with molecular sieves (MS) to sequester the trace amount
of moisture which might be remained or introduced during the
experiment even with the protection of argon or nitrogen. We
suspected that it was the moisture sequestered by the molecular
sieves (namely H2O/MS) that took part in the in situ donor (1f) was slightly more reactive than its ‐counterpart
(1f).9b And the moderate / selectivity is in accordance with that
attained from the C‐allylation of the corresponding
arabinofuranosyl acetate and fluoride donors.4d,e C‐Glycosylation
with 3,4,6‐tri‐O‐benzyl‐2‐deoxy‐glucopyranosyl donor 1g furnished
the desired allyl C‐glycoside 3g in excellent yield (87%) and 
selectivity (/ = 13:1) (entry 6). In comparison, the 2‐deoxy‐
galactopyranosyl donors 1h were more reactive; the reactivity
protodeauration reaction. Thus, we dried the CH2Cl2 solution of the
reactants 1b and 2a and the CH2Cl2 solution of Ph3PAuNTf2 with 4Å
MS, respectively, and took only the supernatants for reaction. The
reaction stopped at ~13% conversion of donor 1b into C‐glycoside
3b. This result supported the proposed role of the H2O/MS in the
present C‐glycosylation reaction. In addition, the resulting –OH
might consume the Me3Si+ derived from the allylation of 2a to
2 | Chem.Commun. 2015, 00, 1‐4
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high
stereoselectivity
being
only
attained
DOI: 10.1039/C6CC07218F
with
1‐
difference between the ‐ and ‐anomers 1h and 1h was not
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discerned in the glycosylation, both led to allyl C‐glycoside 3h in styrenyloxytrimethylsilane (2c) (/ = 5.4:1 and 6.2:1 for 4e and 4i,
respectively), which is more nucleophilic than 2d‐2f.20c
quantitative yield and complete  selectivity (entries 7 and 8).18 The
glycosylation of the ‐ and ‐anomer of 3,4‐di‐O‐benzoyl‐2‐deoxy‐
ribopyranosyl ortho‐hexynylbenzoate (1i and 1i) led to allyl C‐
glycoside 3i in a similar yield of 87% with high  selectivity (entries
9 and 10).4f,h However, the reaction with the ‐anomer as donor
resulted in a higher / ratio (20:1) than that with the ‐
counterpart 1i (/ = 7:1), this result implied the involvement of
an early intermediate,19 such as the glycosyloxypyrylium gold(I)
complex III, in the C‐glycosylation step.
Table 2. C‐Glycosylation of armed glycosyl ortho‐hexynylbenzoates
(1c‐1i) with allyltrimethylsilane (2a) under the catalysis of
Ph3PAuNTf2.
nBu
SiMe3
2a
O
O
OABz
ABz =
O
Ph3PAuNTf2 (0.1 equiv);
O
CH2Cl2, 4A MS, RT, 30 min.
3c-3i
ratio)
1c-1i
entry
donor
OABz
product (yield, /
RO
RO
O
O
3c (R = MBz) (80%, only)
3d (R = Tol) (99%, only)
1c (R = MBz)
1d (R = Tol)
1
2
RO
RO
OBzM
O
OBzM
MBzO
BnO
MBzO
3e (33%,
3e (88%,
/
/
= 1:2)
O
OABz
= 1:3)a
3
1e
Scheme 3. C‐Glycosylation of armed glycosyl ortho‐
hexynylbenzoates (1b, 1g, and 1j) with silyl enol ethers (2c‐2f)
BnO
O
BnO
O
BnO
3f (80%,
3f (87%,
/
/
= 1:2.5)
= 1:2.5)
4
5
1f
a
OABz
under the catalysis of Ph3PAuNTf2. Reaction conditions: donor 1
1f
(1.5 equiv), silyl enol ether 2 (1.0 equiv), Ph3PAuNTf2 (0.1 equiv),
toluene, 5Å MS, argon, RT, 30 min. b Reaction conditions: donor 1
(1.0 equiv), silyl enol ether 2 (1.2 equiv), Ph3PAuNTf2 (0.1 equiv),
CH2Cl2, 4Å MS, argon, RT, 30 min.
BnO
BnO
OBn
O
OBn
O
BnO
BnO
BnO
BnO
3g (87%,
/
= 13:1)
1g
6
OABz
OBn
OBn
OBn
O
OBn
O
7
8
1h
1h
3h (99%, only)
3h (99%, only)
OABz
OABz
In conclusion, C‐glycosylation of glycosyl ortho‐hexynylbenzoates
with allyltrimethylsilane or silyl enol ethers could proceed smoothly
under the catalysis of Ph3PAuNTf2. Depending on the coupling
partners, the C‐glycosylation could be highly stereoselective to
provide in high yields the corresponding allyl, 2‐carbonyl, or enonyl‐
C‐glycosides, which are versatile building blocks for the synthesis of
various C‐glycosides of biological significance.21 The mechanism of
the present gold(I)‐catalyzed C‐glycosylation reaction has been
proposed on the basis of a series of control experiments. It is
disclosed that the moisture which is sequestered by the molecular
sieves plays a key role in the re‐generation of the catalytic gold(I)
species. This finding shall be valuable in understanding the
mechanism underlining other glycosylation protocols, wherein the
moisture, which is previously thought to be removed by the
molecular sieves, might be released to play an important role.
BnO
BzO
BnO
BzO
O
O
9
1i
1i
3i (87%,
3i (87%,
/
/
= 7:1)
10
= 20:1)
OBz
OBz
aThe reaction was performed at ‐72 oC.
Next, we explored the gold(I)‐catalyzed glycosylation with a panel
of the silyl enol ethers (2c‐2f)20 as C‐nucleophiles (Scheme 3).
Subjection of perbenzyl glucopyranosyl donor 1b and 1‐
styrenyloxytrimethylsilane 2c (1.2 equiv) to the previous allylation
conditions (0.1 equiv Ph3PAuNTf2, 4Å MS, CH2Cl2, RT, 30 min) led to
the desired C‐glycoside 4a in only moderate yield (37%) and
stereoselectivity (/ = 2.5:1). Nevertheless, replacing the solvent
CH2Cl2 with toluene and 4Å MS with 5Å MS, the reaction of 1b (1.5
equiv) with 2c proceeded smoothly to provide 4a in excellent yield
(93%) and  selectivity (/ = 10:1). Under this modified condition,
the glycosylation of 1b with 4‐methyl‐2‐trimethylsilyloxy‐1‐pentene
2d remained the high  selectivity (/ = 10:1) in affording C‐
glycoside 4b in 77% yield. However, the similar reactions with (2,2‐
Financial support from the Ministry of Sciences and Technology
of China (2012ZX09502‐002) and the National Natural Science
Foundation of China (21432012 and 21372253) is gratefully
acknowledged.
dimethyl‐6‐methylene‐6H‐1,3‐dioxin‐4‐yloxy)trimethylsilane
(2e)
and 1‐(trimethylsiloxy)‐1‐methoxy‐1,3‐butadiene (2f) were found to
be poorly stereoselective, providing the coupled C‐glycosides 4c
(65%) and 4d (80%) with / ratio of 1.2:1 and 1.5:1, respectively.
The glycosylation of these silyl enol ethers (2c‐2f) with perbenzyl 2‐
deoxy‐glucopyranosyl donor 1g and 2‐deoxy‐ribofuranosyl donor 1j,
which are more reactive than 1b, proceeded smoothly under the
previous C‐allylation conditions. All the reactions provided the
desired C‐glycosides (4e‐4l) in high yields (70%‐96%), however, with
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8332; (b) Y. Tang, J. Li, Y. Zhu, Y. Li and B. Yu, J. Am. Chem.
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J. 2015, 21, 8771–8780.
8
9
10 A proton source is obviously presented in the previously
reported gold‐catalyzed allylations with
allyltrimethylsilane/silyl enol ethers, see for examples: (a) B.
Padhi, D. S. Reddy and D. K. Mohapatra, RSC Adv. 2015, 5,
96758–96768; (b) S. T. Staben, J. J. Kennedy‐Smith, D. Huang,
B. K. Corkey, R. L. LaLonde and F. D. Toste, Angew. Chem. Int.
Ed. 2006, 45, 5991–5994; (c) T. Iwai, H. Okochi, H. Ito and M.
Sawamura, Angew. Chem. Int. Ed. 2013, 52, 4239–4242.
11 A gold(III)‐catalyzed glycosylation with allyltrimethylsilane
was reported to give the allyl C‐glycoside in moderate yield,
see: S. R. Koppolu, R. Niddana and R. Balamurugan, Org.
Biomol. Chem. 2015, 13, 5094–5097.
12 Asao et al. disclosed that reactions of ortho‐alkynylbenzoic
acid alkyl esters with silyl enol ethers in the presence of a
gold(I) catalyst led to the alkylated silyl enol ethers, see: (a) H.
Aikawa, T. Kaneko, N. Asao and Y. Yamamoto, Beilstein J. Org.
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